1. Field of the Invention
The present invention relates to a multilayer superconducting wire composed of a substrate of long length, such as a tape, onto which a layer of super-conducting material is deposited.
In particular the present invention relates to such a multilayer superconducting wire also referred to “coated conductor” and having an essentially round cross section.
2. Related Art
Superconducting wires are known to carry DC currents of extraordinary high current density with very low losses. For many application in the energy industry there is a need to carry large AC currents of typically 50 Hz or 60 Hz frequency.
There are a number of known loss mechanisms in superconductors, which effectively lead to dissipation of heat in a superconducting wire. As superconductors are used at very low temperatures (typically 4.2 K, liquid Helium, for Low Temperature Superconductors, “LTS”, or up to 77 K, the temperature of boiling liquid nitrogen, for High Temperature Superconductors, “HTS”), any dissipation is penalized by the need to use a refrigerator to pump the heat losses from cryogenic temperature to room temperature. Dissipation of 1 W at 77 K typically requires a 10 W power input of a refrigerator to remove these losses.
Nowadays there are known a plurality of different configurations of superconductors, the configuration being varied in view of the intended application field.
For example, DE 197 24 618 A1 discloses a tubular superconductor composed of a support layer made of a metal, an adhesion layer and provided onto the adhesion layer a metal foil onto which is applied a superconducting layer. The multilayer tube has a corrugated wave like shape and can be used in high frequency cables.
German utility model G 91 11 328 U1 relates to a superconductor tape wherein onto a metal support tape a number of individual superconducting filaments are arranged in parallel to each other and extending along the length of the metal support tape. The individual superconducting filaments are electrically connected to each other by superconducting contacts which are provided in distinct intervals and which bridges adjacent filaments. According to one embodiment it is proposed to wind the assembly helically onto a core.
DE 100 45 065 A1 relates to a process for applying a superconducting layer onto a metal carrier tape by pulsed laser deposition. For enhancing the deposition efficiency it is suggested winding the metal carrier tape onto a metal pipe during deposition, wherein the metal pipe is rotated when passing the deposition chamber.
US 2005/0181954 A1, generally, relates to superconducting cables composed of coated conductor tapes which can be helically wound around a support mandrel. The coated conductor tapes are composed of at least a metal carrier tape, one or more buffer layers and a superconducting layer. According to some embodiments stacks of such coated conductor tapes are used, wherein two or more coated conductor tapes are stacked on top of each other.
In many potential applications high currents are needed, often at high magnetic field levels. Examples include superconducting high or medium voltage power transmission cables, fault current limiters, transformers, magnets (e.g. for magnetic resonance imaging) and synchronous motors with superconducting windings. Even in applications with DC currents (DC magnets, synchronous motors), losses occur during field changes, when currents are ramped sometimes very quickly. These losses can be so high, that heat build-up in a component heats the winding to above its safe operating limit causing the device failure. AC losses are therefore the key bottleneck to the wider application of HTS superconductors, often limiting the economic justification of superconductors or even completely rendering an application impossible.
AC loss mechanisms in superconductors are well understood (see, e.g., M. N. Wilson, Superconducting Magnets, Clarendon Press, Oxford, 1983, ISBN 0-19-854810-9, pages 159-197). Three major mechanisms are:
(1) Eddy current losses in normal metals caused by time varying magnetic fields. Normal metal is used in every technical superconducting wire as a base or matrix material stabilising a superconductor mechanically and electrically.
(2) Hysteretic losses in the superconductor are caused by magnetic flux lines (vortices) entering and leaving the superconductor as it swings through an AC cycle. In a simplified model losses are proportional to the change in magnetization ΔM˜Jc×r, where Jc is the critical current, and r is the characteristic size of superconductor. The hysteretic losses can be reduced by breaking up the superconductor into small filaments, and bundling the filaments in a metallic matrix. This approach is well known and efficient in Low Temperature Superconductors (LTS), whereas, for example, thin filaments (from submicron thickness up to about 100 μm, depending on application) of Niobium-Titanium wire are embedded in a Copper matrix. This is called a composite wire.(3) Coupling losses occur in composite wires due to eddy currents in the matrix effectively short circuiting the superconducting filaments. In effect this very much reduces the positive effect of splitting up a superconductor into filaments in the first place. As an effective countermeasure against coupling losses, the composite wire is twisted along its length with a twist pitch on the order of centimeters. Thus filaments change position in the ac field and the inductive voltage caused by eddy currents in the superconductor is changing sign with twist pitch length, effectively zeroing out this loss contribution.
While twisting works well in ductile metallic superconductors (mostly LTS), ceramic oxide superconductors based on the most promising composition YBCO (YBa2Cu3O7, or more generally ReBCO, where Re stands for a rare earth element) cannot be drawn into wires but are fabricated by layer deposition processes on planar carrier tapes, usually metallic carrier tapes.
FIG. 1 shows an example of a state-of-the-art tape. The carrier tape 1 (e.g. stainless steel, Hastelloy, Ni or NiW alloys) is coated with buffer layers 2 (typically a sequence of oxide insulator layers such as MgO, yttria stabilized zirconia (YSZ), CeO2, La2Zr2O7 (LZO) or other in a plurality of physical or chemical deposition processes (see, e.g., M. P. Paranthaman and T. Izumi, MRS Bull. 29, 533-536 (2004)). The buffer layer(s) protecting HTS from interactions with metallic carrier, are grown to have a texture of its crystal lattice matching single crystalline, a-b crystal axis oriented YBCO as closely as possibly.
As the next step, YBCO layer 3 is coated onto the buffer 2 again in a wide variety of physical or chemical deposition processes, e.g. metalorganic chemical vapour deposition (MOCVD), thermal evaporation, metal organic deposition (MOD), and other.
In a technical conductor very often further layers 4 are deposited (e.g. metals like Cu, Ag, Au, typically by electro-coating) that serve to protect the YBCO layer against mechanical, chemical or electrical damage (e.g. as a shunt) in a given application. The shunt layer typically protects the conductor against thermal run-away and burn-out in case that one part of the superconductor became normal and developed a high resistance.
Such tape-like thin film superconductor—often also referred to “coated conductors” or “second generation HTS wire”—and processes for manufacturing are well known in the art and are widely described.
The problem here is that, typically buffer layer(s) and ceramic oxide superconductors such as YBCO cannot withstand mechanical deformation beyond narrow limits without damage.
There have been various attempts in the art to reduce AC losses in tape conductors. FIG. 2 shows an example, where the superconductor layer has been broken up into four strips or filaments 5 to reduce hysteretic losses. A metallic shunt layer 4 is coating the outside of the conductor (FIG. 3). Unfortunately, this solution does not effectively reduce losses, as coupling losses through the connective coating remain dominant.
It has been proposed to twist the tape around its length (C. E. Oberly et al., Cryogenics 41, 117-124 (2001)). As a practical conductor suitable for winding a coil this is ineffective, as the twists build up conductor thickness like a tie and thus do not fill space efficiently and make winding a coil or other components therefrom nearly impossible.
In WO2006/023826 A2 the tape is structured such that filamentary lines of superconductor run from one side of the tape to the other in a shallow angle. In order to return to the other side, mimicking twist, a short superconducting link is bridging over all intermediate filaments in a second layer with insulation in between. This, being an efficient reduction of losses, is a challenging production process.
In a further known solution disclosed in WO2005/096322 A1, a wide superconductor tape is cut into smaller width in an elongated zig-zag pattern. A number of these tapes are then formed into the so-called Roebel cable. While this reduces losses in a wide tape efficiently, the width of each tape is still a couple of millimeters, thus not reducing losses far enough for many applications.
It was the object of the present invention to provide such a superconducting wire wherein the superconducting wire has reduced AC losses and is obtainable in a simple and efficient manner.
This object is solved by a superconducting wire composed of a tubular carrier tape provided with a HTS layer wherein the HTS layer is fabricated as a spiral running around the longitudinal axis of the superconducting wire and wherein at least one buffer layer is provided between tubular carrier tape and HTS layer.
In particular, according to the present invention the superconductor layer is composed of one or more stripes which run helically around a tubular substrate, that is the superconductor layer is applied onto the tubular substrate in helical configuration.
It is the intention of the invention to reveal the geometry of the superconducting wire as well as methods to produce the wire efficiently.
Typically, the carrier tape is a metal or metal alloy tape. The tape can be textured such as obtainable by the well-known RABIT process.
According to need one or more buffer layer(s) can be provided between the carrier tape and the HTS layer. Suitable metals, metal alloys and buffer layer(s) are generally known and can be used for the present invention.
For the carrier tape as well as for the buffer layer(s) those referred to above with respect to the prior art can be used.
Generally, any ceramic oxide HTS can be used for the present invention.
The HTS material may be of ReBCO-type with Re being at least one of selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Y, Tm, Yb and Lu with Y being preferred. Preferably, the HTS material is ReBa2Cu3O7 with Re being as defined above.
The buffer layer(s) and HTS layer typically are provided on the outward face of the tubular carrier tape. The HTS layer is patterned into one or more screw lines, in the following also referred to “lane”, snaking around the length of the wire with a width and twist pitch that in can be optimized according to the application.
In particular, with the present invention it is possible to obtain lanes of superconductor material with very small width.